A method for attaching a humidity measurement sensor to an intake part of an internal combustion engine and controlling the internal combustion engine by processing the information with an electronically controlled control device is in wide use as an exhaust gas and fuel economy improvement. In addition, there are various principles regarding air humidity measurement methods, and examples thereof include a thermal humidity sensor using heat radiation as in PTL 1 and a relative humidity sensor disclosed in PTL 2 in which a dielectric constant changes as a result of moisture infiltration into a polymer and it is measured as capacitance.
Grasping the absolute amount of moisture, that is, an absolute humidity is important in controlling the internal combustion engine. The thermal humidity sensor directly measures the absolute humidity. In a case where the relative humidity sensor is used, however, the absolute humidity is calculated from a relative humidity and the temperature of that place. In addition, various methods have been devised as in PTL 3 and PTL 4 with regard to where the humidity sensor is to be attached in the internal combustion engine.